This invention relates generally to a load cell for the measurement of force. More specifically, the invention relates to a load cell for the measurement of force resulting in strain or stress created by means such as, for example, force, acceleration, or pressure converting the force into an electronic signal transmitted to a point of computation or evaluation. The device resists shock and the damage from shock by resisting the generation of forces from the acceleration of internal masses in the direction of sensitivity on the sensing elements.
Load measuring devices and cells are known in the art. For example, Johnson, U.S. Pat. No. 5,313,023 using force sensors within which move with the mass of the springs attached to them limiting the maximum tolerable acceleration. Gallo, U.S. Pat. No. 4,043,190, discloses a meter for measuring mass or force wherein the sensed displacement acts indirectly on the tension of the two transversely vibrating electrically excited strings with a pretension mass. Blawert et al, U.S. Pat. No. 4,237,988, similarly disclose an overload protection device for precision scales where the force sensing element is free to move from the deflection caused by the applied load. Paros, U.S. Pat. No. 4,384,495 discloses a mounting structure for double bar resonators to ensure symmetrical loading of the resonator responsive directly to external forces.
Further, Streater et al, U.S. Pat. No. 3,712,395, disclose a weight-sensing cell which includes two differentially loaded vibrating members where attached masses move with the members. The prior art load cells were dependent on the stability of the loaded structure, for output stability. For example, Albert, U.S. Pat. No. 4,838,369 discloses a load cell intended to provide a linear relationship between the signal generated and the force sensed. Albert relies on a longitudinally rigid structure to resist interference from varying load positions.
The force sensors in the above conventional load cells were sensitive to force. In particular, forces caused by acceleration of the masses attached to the force sensors would cause force on the sensors not limited by the deflection stop of the load cell. Any excessive accelerations due to impact to the load cell from rigid objects or the scale falling a distance (a shock) would cause the force sensors to be exposed to force beyond their maximum capacity causing the force sensor to fail. The load cell would be destroyed when a force sensor failed. Therefore, there exists a need for a load cell with impact resistant qualities and it is desirable to provide shock resistant electronic load cell and it is to this end that the present invention is directed.
In accordance with the invention, there is provided a force sensing load cell comprising a three-dimensional structure having an interior opening defined by an upper wall, a lower wall, and first and second side walls, a base affixed to the upper or lower wall within the interior opening of said three-dimensional structure, means for supporting capacity affixed to the base, a connection element attached to the opposite wall spaced apart from, and parallel in at least one plane to the means for supporting capacity and means for sensing force, the force sensing means affixed between the connection element and the capacity supporting means. The means for supporting capacity and the connection element function as springs. A spring is an element which may store energy through deflection when a force does work on the moveable portion of the spring and which may do work by returning stored energy by providing a force moving through a distance. Further embodiments may comprise more than one force sensing means affixed between any number of capacity supporting means and additional load bearing elements acting as springs parallel to the capacity supporting means attached to the connection element.
Reduced shock susceptibility is achieved by placing the center of mass of the movable end of the capacity supporting means attached to the force sensor nearly at or at a distance from the plane defined by the flexing axis of the opposite wall such that the movement of this mass due to load cell deflection is not in the direction of the axis of sensitivity of the sensor. This is achieved by matching the deflection in the direction of the axis of sensitivity of the sensor due to the load cell deflection of the load bearing elements and the capacity supporting means to that of the connection element in that direction due to the same effect. The relationship between the spring constants, flexure spacing, and position of null horizontal motion is defined by the equation:   a1  =            H      2        ·          [                                    k2            -            k1                                k2            +            k1                          +        1            ]      
Where a1 is the vertical position of null horizontal motion from the flexure, H is the vertical spacing between the flexures in the parallelogram, k1 is the connection members spring constant, k2 is the combined spring constant of the load bearing elements, and the capacity supporting means springs.
The above equation will provide a nearly optimum shock resistant load cell design unless the side walls are flexible or the scale support structure deflects. Adjust the spring constants and the null motion position from the theoretical values to optimize shock resistance in the final scale. Small variations from the calculated relationship will empirically optimize the design when the scale assembly survives maximum shock. Finite element analysis will also provide a close to optimum design of the load cell for a specific application. A substantial dead load on the load cell produces a deflection that may require an adjustment to allow maximum shock resistance under the desired loaded condition.
In accordance with one preferred aspect of the invention there is provided a force sensing load cell comprising a three-dimensional structure having an interior opening defined by an upper wall, a lower wall, and first and second side walls, a base affixed to the upper or lower wall within the interior opening of said three-dimensional structure, first and second capacity supporting cantilever beams affixed to the base and extending vertically within the plane of the interior opening, a load bearing cantilever beam affixed to the base between the capacity supporting cantilever beams and extending vertically within the plane of the interior opening parallel to the capacity supporting cantilever beams, a connecting cantilever beam attached to the opposite wall and extending vertically within the plane of the interior opening connected to the load bearing cantilever beam and parallel to the capacity supporting cantilever beams, and first and second electrical force sensors, the first force sensor affixed between the first capacity supporting cantilever beam and the load bearing cantilever beam, the second force sensor affixed between the second capacity supporting cantilever beam and the load bearing cantilever beam. Stress on the load cell from the incidence of a force stresses the two sensors oppositely. Independent signal processing of the first and second electrical sensor outputs produces independent mode signals separate from a differential mode signal and there is nearly no horizontal movement of the sensors due to vertical deflection of the load cell. Vertical impacts to the load cell do not cause the sensors to accelerate horizontally and induce horizontal forces on them as the masses attached to them accelerate.
In accordance with another preferred aspect of the invention there is provided a force sensing load cell comprising a three-dimensional structure having an interior opening defined by an upper wall, a lower wall, and first and second side walls, a base affixed to the upper or lower wall within the interior opening of said three-dimensional structure, a wall opening within said upper or lower wall opposite the base, first and second capacity supporting cantilever beams affixed to the base and extending vertically through the plane of the interior opening and into said wall opening, a load bearing cantilever beam affixed to the base between the capacity supporting cantilever beams and extending vertically across the plane of the interior opening parallel to the capacity supporting cantilever beams, a connecting member attached to the opposite wall connected to the load bearing cantilever beam, and first and second electrical force sensors, the first force sensor affixed between the first capacity supporting cantilever beam and the load bearing cantilever beam, the second force sensor affixed between the second capacity supporting cantilever beam and the load bearing cantilever beam. Stress on the load cell from the incidence of a force stresses the two sensors oppositely. Independent signal processing of the first and second electrical sensor outputs produces independent mode signals separate from a differential mode signal and there is nearly no horizontal movement of the sensors due to vertical deflection of the load cell. Vertical impacts to the load cell do not cause the sensors to accelerate horizontally and induce horizontal forces on them as the masses attached to them accelerate.
Preferably, the structure is machined monolithically from an isotropic metal, therefore, the modulus of elasticity is nearly homogeneous. There is nearly cancellation of the elastic modulus effect if the force sensor and its attachments are very stiff relative to the capacity supporting cantilever beams.
The cantilever beams should be designed for vertical load cell deflection to cause no horizontal motion of the material passing through the plane of the center of mass of the end of the capacity supporting cantilever beams attached to the force sensors.